Protecting aluminum from corrosion



Patented Feb. 6, 1934 UNITED S-TA PROTECTING AL COBRO SION Junius 1).

Aluminum Company of Edwards, Oakmont, Pa., aaaignor to America,Pittsburgh,

Pa., a corporation of Pennsylvania No Drawing. Application April 29,1988, Serial No. 668,828, and

This invention relates to the covering of aluminum and aluminum alloyswith a coating capable of resisting the action of corrosive agencies.The invention particularly contemplates the produc- 5 tion on such metalsurfaces of a corrosion resistant coating of the type known in the artas oxide coatings.

These oxide coatings, as they will be termed in the followingdescription and claims. may be 0 produced by several methods, whichmethods, with few exceptio comprise a reaction, either chemical orelectro-chemical, between the aluminum or aluminum alloy and a solutionof a chemically active substance or substances. The coatingsproduced onaluminum by these various and well known methods have ascribed to themcertain properties and advantages, one of which is alleged to be theprotection against corrosive agencies afforded to the coated metal.However, only a few of the oxide coatings so produced afford anysubstantial protection to the aluminum on which they are formed and eventhe best of such coatings leave much to be desired in their "protectiveeffects.

It is accordingly an object of this invention to provide certain methodsof covering aluminum and its alloys with protective coatings having ahigh resistance to the action of corrosive substances and particularlyto salt solutions. A further object of this invention is the provisionof an oxide coated aluminum article of good corrosion resistance andhaving other desirable and novel properties as will hereinafter. appear.

For the purpose of this specification and the appended claims, aluminumor aluminum alloys regardless of the purity of the metal or the natureof the alloying element, will be inclusively described by the genericterm aluminum".

l. have discovered as the result of an extensive so investigation thatan adsorbent oxide coating when formed on aluminum may be thereaftertreated by a simple process which greatly increases the emciency of theoxide coating as a protective covering against corrosive agencies. Animportant advantage of the process is that the corrosion resistance isnot less in one area or part of the coating than another but issubstantially equal over the entire coated surface. 50 My novel methodcomprises, as a first and neces= sary step, the formation on thealuminum of an oxide coating having adsorptive properties and, as asecond step, the treatment of the coated metal with a solution or asoluble substance, 55 namely chromic acid, inhibitory to corrosive tackby chromic acid, and

to strip the coating.

. little attack on the in Canada June so. 193 3 Claims. (01. 1486)agencies and capable of being uniformly adsorbed in and on theadsorptive oxide coating.

I have found, however, that oxide coatings on aluminum, howeverproduced, are subject to atmay be virtually if not M completelydestroyed thereby, especially when the acid solution is hot. Forexample, a 0.3 mil oxide coating on duralumin may be completely strippedby immersing the coated metal in a 5 per cent solution of chromic acidat C. for 5 about 6 minutes, whereas 20, 45 and minute immersions arerequired in 5 per cent chromic acid solutions at 40, 30 and 20 C.,respectively, On the other hand, with 1,

5 and 10 percent solutions of chromic acid at 1 40 C., the times req edto dissolve a 0.3 mil coating on duralumin are about 25, 20 and 17minutes, respectively. Concentrations as low as 0.1 per cent areeffective for my process and cause film. In cases where the oxide 75film is not completely stripped from the metal, the surface of what isleft of the film may be rendered soft and powdery by the use of achromic acid solution at too high a concentration or temperature. Byadjustment of concentration, temso perature and time of treatment, theimpregnating treatment can be effected without material impairment ofthe film. In general I prefer a concentration of not more than about 10per cent, and a temperature not higher than about. 35 35 C., as givingsatisfactory results in practice with an immersion of not more thanabout 20 minutes. By a few trials at given concentration or temperaturea suitable time of treatment can be readily found. In some cases a timewhich no will not impair the film may be too short to produce' thedesired impregnation and in such cases the temperature or concentration,or both, can

be adlusted.

I have further found that even though an adsorbent oxide coating beimpregnated with chromic acid without being seriously attacked, and bethen washed, the retained chromic acid which the coating had taken upreadily washes out; but that if after removal from the impregnatingsolution the coating is thoroughly dried without washing a betterretention of the acid is obtained. The protectiveefiect of the coatingis thus substantially increased, even under such severe conditions asexposure to salt spray for extended periods. I am unable to say whatstabilizing change is caused or brought about by the drying, but iteffects a marked improvement in corrosion resistance. In preparingoxidecoated aluminum articles in mi accordance with my invention it isdesirable that the coating be comparatively heavy or thick, but acomparatively thin coating may be satisfactory, especially' if itsadsorptive power is high. ,To the preparation of such oxide coatingscertain methods are peculiarly adapted. My preferred method consists inmaking the aluminum the anode of.

an electrolytic cell, the cathode of which may be aluminum, lead orother metal, and using as the electrolyte in such cell an aqueoussolution of sulphuric acidr The aluminum anode and the cathode'areimmersed in such an electrolyte and external electrical energy isimpressed upon the electrodes with the result that a layer of oxide isformed on the aluminum anode surface which is adherent, hard and denseand possesses remarkable adsorptive powers. The concentration ofsulphuric acid may vary within very wide limits, such as 1 to per cent,and good results will be obtained, but I have preferred to use solutionscontaining between -5 to 10 per cent of sulphuric acid. I also prefer touse a current density of about 0.1 to 0.4 amperes per square inch ofanode surface. Other methods of forming these oxide'coatings are wellknown, and may be used, but I prefer the method outlined above.

The coating must be firm, adherent and resistant to abrasion, and hencesuch coatings as are produced by water or by atmospheric oxygen areexcluded when I speak of oxide coatings.

The aluminum, having been provided with an adsorptive oxide coating, isnow preferably washed with water to remove residual solution remainingfrom the coating treatment, and is then treated, in accordance with myinvention, with the chromic acid solution. This treatment may takevarious forms but I prefer immersion of the oxide-coated aluminumarticle in an aqueous solution of the acid, after which the article isdried without washing.

As a specific example of my method of coating aluminum with a corrosionresistant coating and the resultsobtained thereby, there may be citedthe treatment of aluminum spandrels which are designed for use on theouter walls of buildings. Cast articles of this nature made of analuminum base alloy containing about 5 perIcent of silicon were madetheanode of "a cell the electrolye of which was a solution of 7 per centsulphuric acid. In 20 minutes a. highly adsorbent oxide coating of graycolor was formed on the aluminum alloy castings. The coated castingswere then washed and dried, and immersed in a 4 per cent chromic acidsolution at room temperature (20 C.) for -.a period of 10 minutes. Atthe end of this period they were removed from the solution and driedwithout washing. The-resulting coated articles were then tested byimmersion for a period of 72 hours in a highly corrosive solutioncontaining about 7.5 per cent of sodium chloride and about 3 per cent ofhydrogen peroxide. An inspection of the castings at the end of thatperiod revealed that the coating on their surfaces had completelyresisted the corrosive action of the sodium' chloride-hydrogen peroxidesolution. At the end of a further period of 72 hours in a corrosivesolution of the same composition the same articles were only slightlyaffected. Similar oxide coated aluminum castings treated in accordancewith my invention exhibited no signs of corrosion after an exposure ofabout 5 months to the atmosphere.

This application is a continuation in part of my copending applicationSerial No. 472,744, filed August 2, 1930.

I claim:

- 1. A method of protecting aluminum surfaces against corrosion,comprising depositing on the aluminum surface an adsorptive oxidecoating, thereafter impregnating said oxide coating with a chromic acidsolution, and drying the impregnated coating without washing.

2. A method of protecting aluminum surfaces against corrosion,comprising forming on the aluminum surface an adsorptive oxide coating,

thereafter treating said oxide-coated surface 3. A method of protectingaluminum surfaces against corrosion, comprising forming on the aluminumsurface an adsorptive oxide coating; thereafter impregnating said oxidecoating sur- ;face with a chromic acid solution of not more i .20 than10'per cent strength at a temperature not higher than 35. C. and'for aperiod of not more than about 20 minutes; and drying the impregnatedcoating without washing.

JUNIUS D. EDWARDS.

